This is a study of the mechanism of age-related changes in calcitriol (CTL) and parathyroid hormone (PTH) metabolism in adults. It aims specifically to determine, in elderly men and women, the effect of normalizing age-related declines in the serum concentrations of insulin-like growth-factor-I and growth hormone on the serum concentrations of PTH and CTL, and the production and metabolic clearance rates of CTL.